1. Field of the Invention
This invention relates in general to electrodeless gas discharge lamps and, in particular, to drive circuits for such lamps that use alternating magnetic fields to produce a plasma discharge in the lamps.
2. Description of the Related Art
Radio frequency drive circuits for electrodeless gas discharge lamps sometimes utilize an inductive drive coil to produce a plasma discharge within the lamp envelope. Alternating current flow through the coil generates an alternating magnetic field that impinges on the ionizable gas fill within the lamp, thereby producing the plasma discharge. These drive coils may be helically wound about the lamp envelope, as in U.S. Pat. No. 4,902,937 to Witting. Alternatively, the lamp envelope may include a central recessed portion within which the drive coil is located, as in U.S. Pat. No. 4,797,595 to De Jong. As shown in the De Jong patent, the drive coil can be wound around a magnetically permeable core which has the effect of increasing the inductance of the drive coil.
In applications such as automotive vehicle lights where operating power comes from a battery, it is desirable to minimize the power used to operate the lamps. However, external vehicle lights such as tail lights must produce sufficient intensity to accommodate the various ambient lighting conditions that can be encountered in normal use. Consequently, it is desirable to increase the efficiency of the lamp drive circuit, so that power consumption can be reduced without a commensurate reduction in light output from the lamp.
Accordingly, it is an object of this invention to increase the efficiency of electrodeless gas discharge lamp drive circuits by improving the coupling of the magnetic field to the gas fill within the lamp. It is also an object of this invention to reduce the strength of the magnetic field at locations external to the lamp so as to minimize the potential interference of the lamp drive circuit with other electronic circuits.
In accordance with the present invention there is provided an electrodeless gas discharge lamp assembly that includes a gas discharge lamp having an envelope containing an ionizable gas fill, an inductive drive coil having a number of turns of an electrical conductor wound about the lamp envelope, and a flux concentrator comprising a magnetically permeable material disposed about at least a portion of the drive coil and lamp envelope. The flux concentrator can comprise a tubular sleeve which can have an axial split that extends the length of the sleeve. The sleeve operates to confine the magnetic flux lines to thereby reduce the amount of magnetic field emanating outside the lamp assembly. To reduce eddy current losses, the flux concentrator can be formed from electrically isolated laminations of the magnetically permeable material.
The flux concentrator can include a magnetically permeable end piece that is integrally attached to one end of the sleeve. This helps to further confine the magnetic flux lines at the one end of the sleeve. The flux concentrator can also include a magnetically permeable core piece that is integral with the end piece and that extends into a recessed portion of the lamp envelope. This core piece concentrates the magnetic flux lines through a central portion of the lamp where the plasma discharge is primarily located.